150 Royal Institution : — 



Cause these gases to combine chemically and form nitrous oxide, hoth 

 the absorption and radiation are thereby augmented 250 times ! 



In this way we look with the telescope of the intellect into atomic 

 systems, and obtain a conception of processes which the eye of sense 

 can never reach. But gases and vapours possess a power of choice 

 as to the rays which they absorb. They single out certain groups of 

 rays for destruction, and allow other groups to pass unharmed. This 

 is best illustrated by a famous experiment of Sir David Brewster's, 

 modified to suit the requirements of the present discourse. Into a 

 glass cylinder, with its ends stopped by discs of plate glass, a small 

 quantity of nitrous acid gas was introduced, the presence of the gas 

 being indicated by its rich brown colour. The beam from an electric 

 lamp being sent through two prisms of bisulphide of carbon, a spec- 

 trum 7 feet long and 18 inches wide was cast upon a screen. In- 

 troducing the cylinder containing the nitrous acid into the path of 

 the beam as it issued from the lamp, the splendid and continuous 

 spectrum became instantly furrowed by numerous dark bands, the 

 rays answering to which were struck down by the nitric gas, while 

 it permitted the light which fell upon the intervening spaces to pass 

 with comparative impunity. 



Here also the principle of reciprocity, as regards radiation and 

 absorption) holds good ; and could we, without otherwise altering its 

 physical character, render that nitrous gas luminous, we should find 

 that the very rays which it absorbs are precisely those which it would 

 emit. When atmospheric air and other gases are brought to a state 

 of intense incandescence by the passage of an electric spark, the 

 spectra which we obtain from them consist of a series of bright bands. 

 But such spectra are produced with the greatest brilliancy when, 

 instead of ordinary gases, we make use of metals heated so highly as 

 to volatilize them. This is easily done by the voltaic current. A 

 capsule of carbon was filled with mercury, which formed the positive 

 electrode of the electric lamp; a carbon-point was brought down upon 

 this ; and on separating one from the other, a brilliant arc containing 

 the mercury in a volatilized condition passed between them. The 

 spectrum of this arc was not continuous like that from the solid carbon 

 points, but consisted of a series of vivid bands, each corresponding 

 in colour to that particular portion of the spectrum to which its ravs 

 belonged. Copper gave its system of bands ; zinc gave its system ; 

 and brass, which is an alloy of copper and zinc, gave a splendid 

 spectrum made up of the bands belonging to both metals. 



Not only, however, when metals are united like zinc and copper 

 to form an alloy is it possible to obtain the bands which belonged to 

 them. No matter how we may disguise the metal — allowing it to 

 unite with oxygen to form an oxide, and this again with an acid to 

 form a salt; if the heat applied be sufficiently intense, the bands 

 belonging to the metal reveal themselves with perfect definition. 

 Holes were drilled in a cylinder of retort carbon, and, these being 

 filled with pure culinary salt, the carbon was made the positive elec- 

 trode of the lamp; the resultant spectrum showed the brilliant yellow 

 lines of the metal sodium. Similar experiments were made with the 



